Among the conventionally-known vehicle occupant protection devices constructed to protect a vehicle occupant when a lateral collision of the vehicle has occurred is one where a side support is provided on a side portion of a seat back of a seat. Such a type of vehicle occupant protection device is known from patent literature 1. In the vehicle occupant protection device known from patent literature 1, the side support protrudes forward from the side portion of the seat back. The side support has an energy absorbing pad provided therein. When subjected to external force, the side surface deforms toward the interior of a passenger compartment of the vehicle and hits the side support. By absorbing the external force by means of the energy absorbing pad, the known vehicle occupant protection device can protect the vehicle occupant seated in the seat.
In recent years, however, there has been a need to further enhance protection performance for protecting the vehicle occupant at the time of a lateral collision of the vehicle.